A new type of chemically functionalized, covalently linked gold nanoparticles, 5 to 40 nm in diameter, will be prepared as improved replacements for colloidal gold. The new particles will be stabilized using a shell of closely-packed alkanethiol ligands: these will be synthetically tailored to import complete water-solubility and maximum biocompatibility upon the stabilized particles. Particle size will be controlled by ligand packing and the ratio of ligands to gold, and different separation methods will be investigated to achieve the narrowest possible size range. Cross-linking of the alkyl chains on adjacent ligands to encapsulate the gold core and increase stability will also be investigated. The modified gold labels will be covalently cross-linked to antibodies and proteins via chemically specific reactive groups incorporated into their coordinated ligands, and also lyophilized for use as labeling reagents. Label properties will be tuned to eliminate non-specific binding, background staining and aggregation which restrict the application of colloidal gold. The new conjugates will be evaluated in parallel with equivalent colloidal gold probes in studies of the functional organization of the mammalian cell nucleus, and the developmental organization of microsporida. Blot testing will be used to tune their properties for future use in rapid diagnostic devices. PROPOSED COMMERCIAL APPLICATION New lsrge gold probes will replace colloidal gold probes and extend their applications. Covalent conjugation will enable the preparation of new reagents with important advantages for microscopy and structural biology which are not feasible with colloidal gold. Their superior design and performance will also enable the development of more sensitive and reliable medical diagnostics and new device formats.